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The present invention relates generally to cooling of circuit boards and, more specifically, to an apparatus and methods used for liquid-cooling circuit boards.
Electronic components mounted on circuit boards generate heat that must be dissipated to assure proper functioning of the components. Air is typically used to cool the circuit board when the total power dissipated is low or when the power density is low. The use of fans, ducting and/or heatsinks to improve air cooling is well understood and widely used in industry.
Air provides insufficient cooling when the total power used is high and in high density power applications because of air""s relatively low thermal capacity. In high power applications, liquid can be used to provide significantly improved cooling, but at an added complexity level, since provision must be made to contain the liquid so it does not contact the components directly.
A way to contain cooling liquid is to use a liquid-cooled cold plate. A liquid-cooled cold plate is typically made of copper, aluminum or their alloys, although other materials can be used. It has channels within it that distribute the cooling liquid and has inlets and outlets that enable the liquid to enter and exit the cold plate. The cold plate is then mated to the electronic circuit board that requires cooling. Electrical components on the circuit board that touch the cold plate are cooled. They become cool because of their close proximity to the cold plate that transfers the heat to the cooling liquid, but at no time do they actually touch the cooling liquid directly.
The fact that cold plates are co-mounted with the circuit boards implies that these cold plates must be removable when the circuit board is removed. Removing the cold plate implies severing the liquid cooling connections. When leaks occur in cold plates, they do not occur within the cold plate but rather occur where the plumbing connections are made to the plate. The plumbing connection is especially challenging when there is a need for the connection to be temporary.
While liquids cool much more effectively than air, systems using cooling liquids are subject to leaking. If a liquid leak occurs in the proximity of an electronic circuit board, it can create a serious problem. This problem can be mitigated by choice of liquid. Water has an excellent heat transfer function, but a water leak can short out electrical components and cause permanent damage. High dielectric fluids, such as fluorocarbons, have been used in liquid cooling to limit the damage caused by a leak. The disadvantages to this approach are that fluorocarbons have a less advantageous heat transfer function than water and fluorocarbons are expensive. In addition, fluorocarbons easily leak through conventional pump seals increasing the number of leaks. In addition, fluorocarbons can still cause damage to electronic systems.
There is a wide variety of cold plate technologies currently in use. Lower performance cold plates commonly use internal metal tubes to distribute the liquid. Higher performance cold plates commonly use vacuum brazed construction. Vacuum brazing allows the use of high performance fins placed within the liquid channel at locations where better heat transfer is required with the cold plate surface.
A further difficulty with cold plate cooling is that the components on a circuit board are seldom at the same height. If the cold plate is substantially planar, only the tallest components will touch the cold plate and be cooled. For small variations in height (xcx9c0.01xe2x80x3), compliant thermal interface material can be applied to lower components allowing the components to touch the cold plate through the interface. However, when the variations are larger, the compliant thermal interface material is not sufficient to bridge the gap.
A cold plate having a xe2x80x9cpersonality profilexe2x80x9d impressed on its side to mate with the component board is one solution to the larger gap issue. This personality profiled cold plate is thicker in places where short components are on the circuit board and is thinner where the taller components are located. Such personalized cold plates must be custom made and can only be used with one type of board. They are difficult to manufacture because the internal liquid path must be adapted to the personality contours of the cold plate. Typically these plates must be inserted and extracted with the circuit board, requiring the use of temporary connections to the cooling liquid source.
A populated printed circuit board cooling system comprises a cold plate and a set of personality plates that assure good contact between the printed circuit board and the cold plate. The cold plate is formed of a heat transmissive material that has internal liquid circulating paths. The cold plate must have an inlet and an outlet port for the circulating liquids. These ports are incorporated in at least one header portion of the cold plate for permanent connection to a liquid circulating source and return. If only one header is used, it is formed as a split header separating the supply and return. The ports may also be split between two headers. The cold plate has a header portion and a flat portion having two flat planar surfaces. The flat planar surfaces may be the opposite sides of a rectangular block or may be joined to form a wedge. The header portion is connected to the top and/or bottom of the flat portion. The remaining sides are sealed to provide a closed environment for the circulating liquid. Each personality plate is formed of a heat transmissive material and has a flat side and a contoured side. The contoured side conforms to the contour of one side of the populated printed circuit board. When the flat surfaces of the cold plate and a personality plate are butted against each other and the side of the printed circuit board is cradled in the contoured side of its personality plate, heat is transferred from components on the printed circuit board through the personality plate to the cold plate to be removed by a circulating liquid.
The heat transmissive materials available include copper, aluminum and their alloys. One or two headers can be used, and each header can have baffles and gates to direct the liquid in a particular circuit through the cold plate. Thermal interface material may be used within the system wherever heat transference is desired. The thermal interface material improves the thermal contact between the heat sources and the cooling mechanisms.
An apparatus for cooling the components on printed circuit boards is made up of a card cage incorporating cold plates and circuit board assemblies that adapt the printed circuit boards to the cold plates and card cage. The plurality of cold plates are each connected to a mechanism that is circulating cooling liquid through a plurality of headers mounted to the top and/or bottom of the cold plates. Each cold plate is formed as a wedge shape with the flat sides oriented towards the sides of the card cage, a narrow end oriented toward the front of the card cage and a wider end oriented toward the rear of the card cage. The cold plates are made of a heat transmissive material.
Each circuit board assembly is custom formed based on its printed circuit board. A pair of personality plates are tailored each to a respective side of the printed circuit board. One side of each personality plate is flat; the other is contoured. The first plate contoured surface is complementary to the contours of a first side of its printed circuit board and the second plate contoured surface is complementary to the contours of the second side of its printed circuit board. The personality plate flat surfaces are angled such that each completed circuit board assembly with personality plates is wedge shaped with a wedge angle complementary to the cold plates"" angle. A fastening mechanism holds the personality plates and printed circuit board tightly pressed together forming the circuit board assembly. The circuit board assemblies are mounted in the card cage with the narrow end facing the rear of the card cage, so that the flat surfaces of the personality plates and their matching cold plate are parallel when installed. Interface material may cover the plate flat surfaces to improve thermal transfer. A plurality of clamping mechanisms, each structured to fully seat a circuit board assembly between a pair of cold plates, press the flat surfaces of the cold plates and the personality plates together forming a thermally conductive path from the components, through the personality plates to the cold plates.
A method of cooling populated printed circuit boards comprises fabricating a card cage incorporating wedge-shaped cold plates that are connected, permanently or otherwise, to a liquid circulating mechanism. The cold plates are arranged to allow board mounting between the cold plates. Printed circuit board assemblies are formed by sandwiching each populated printed circuit board between personality plates. One side of each personality plate conforms to the contours of one side of a particular populated printed circuit board and the other side has a flat surface. The personality plates are made of a heat transmissive material. When the printed circuit board assemblies are inserted into the card cage with the flat sides of the personality plates parallel to the flat sides of the cold plates heat transfer is enabled. By applying force to press the printed circuit board assemblies and cold plates together, the heat transfer is improved and circulating liquid removes the heat from the cold plates. Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows.